Chloric acid - alkali metal chlorate mixtures and chlorine dioxide generation

ABSTRACT

An aqueous solution of chloric acid and alkali metal chlorate is produced in an electrolytic cell having an anode compartment, a cathode compartment, and at least one ion exchange compartment between the anode compartment and the cathode compartment. The process comprises feeding an aqueous solution of an alkali metal chlorate to the ion exchange compartment, electrolyzing an anolyte in the anode compartment to generate hydrogen ions, passing the hydrogen ions from the anode compartment through a cation exchange membrane into the ion exchange compartment to displace alkali metal ions and produce an aqueous solution of chloric acid and alkali metal chlorate, passing alkali metal ions from the ion exchange compartment into the cathode compartment, removing the aqueous solution of chloric acid and alkali metal chlorate from the ion exchange compartment, and, increasing the chlorate ion concentration to provide the aqueous solution of chloric acid and alkali metal chlorate with a total chlorate ion to water molar ratio of about 0.15 or greater.

This application is a division of application Ser. No. 07/786,155, nowU.S. Pat. No. 5,258,105 filed Oct. 31, 1991, which is acontinuation-in-part of application Ser. No. 07/475,603 filed Feb. 6,1990, now U.S. Pat. No. 5,084,148.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for electrochemically producingchloric acid--alkali metal chlorate solutions. More particularly, thisinvention relates to the electrochemical production of chloricacid--alkali metal chlorate solutions suitable for the generation ofchlorine dioxide.

2. Brief Description of the Prior Art

Chlorine dioxide has found wide use as a disinfectant in watertreatment/purification, as a bleaching agent in pulp and paperproduction, and a number of other uses due to its high oxidizing power.There is a variety of chlorine dioxide generator systems and processesavailable in the marketplace. Most of the very large scale generatorsemployed, for example, in pulp and paper production, utilize an alkalimetal chlorate salt, a reducing agent, and an acid in a chemical processfor producing chlorine dioxide. These generators and the processesemployed also produce by-product salts such as sodium chloride, sodiumsulfate, or sodium bisulfate. In pulp and paper mills, the typicalby-product is sodium sulfate (saltcake) which is converted into a sulfursalt of sodium in a high temperature boiler and used in the paperprocess. Boilers require energy and the paper mills have a limitedboiler capacity. Increasing the production of chlorine dioxide generallymeans increased capital investment to provide the added boiler capacityrequired to process the added amounts of saltcake by-product produced.

Thus a process which reduces the amount of a by-product salt, such assodium chloride or sodium sulfate, produced while efficiently generatingchlorine dioxide is commercially desireable.

U.S. Pat. No. 3,810,969 issued May 14, 1974 to A. A. Schlumbergerteaches a process for producing chloric acid by passing an aqueoussolution containing from 0.2 gram mole to 11 gram moles per liter of analkali metal chlorate such as sodium chlorate through a selectedcationic exchange resin at a temperature from 5° to 40° C. The processproduces an aqueous solution containing from 0.2 gram mole to about 4.0gram moles of HClO₃. This process requires the regeneration of thecationic exchange resin with acid to remove the alkali metal ions andthe treatment or disposal of the acidic salt solution.

K. L. Hardee et al, in U.S. Pat. No. 4,798,715 issued Jan. 17, 1989,describe a process for chlorine dioxide which electrolyzes a chloricacid solution produced by passing an aqueous solution of an alkali metalchlorate through an ion exchange resin. The electrolyzed solutioncontains a mixture of chlorine dioxide and chloric acid which is fed toan extractor in which the chlorine dioxide is stripped off. The ionexchange resin is regenerated with hydrochloric acid and an acidicsolution of an alkali metal chloride formed.

In U.S. Pat. No. 4,683,039, Twardowski et al. describe a method forproducing chlorine dioxide in which the chlorine dioxide is produced ina generator by the reaction of sodium chlorate and hydrochloric acid.After separating chlorine dioxide gas, the remaining sodium chloridesolution is fed to a three-compartment cell to form sodium hydroxide andan acidified liquor which is returned to the chlorine dioxide generator.

Each of the above processes produces a fixed amount and type ofby-product salt.

M. Lipsztajn et al. teach an electrolytic-dialytic process for producingchloric acid and sodium hydroxide from sodium chlorate. Chlorate ionsare transferred through an anion-exchange membrane and sodium ions arepassed through a cation-exchange membrane (U.S. Pat. No. 4,915,927, Apr.10, 1990).

BRIEF SUMMARY OF THE INVENTION

Now a process has been discovered which permits variability in thecomposition of a chlorate solution used in chlorine dioxide generators.Further, the process permits a reduction in the amount of acid requiredand subsequently the amount of salt by-product produced in the chlorinedioxide generator. Still further, the process allows for the productionof an alkali metal hydroxide as a valuable by-product or acidicsolutions of alkali metal salts at reduced energy costs. In addition,the process results in the reduction of process steps and processequipment required for the production of chlorine dioxide.

These and other advantages are accomplished in a process forelectrolytically producing an aqueous solution of chloric acid andalkali metal chlorate in an electrolytic cell having an anodecompartment, a cathode compartment, and at least one ion exchangecompartment between the anode compartment and the cathode compartment,characterized by feeding an aqueous solution of an alkali metal chlorateto the ion exchange compartment, electrolyzing an anolyte in the anodecompartment to generate hydrogen ions, passing the hydrogen ions fromthe anode compartment through a cation exchange membrane into the ionexchange compartment to displace alkali metal ions and produce anaqueous solution of chloric acid and alkali metal chlorate, passingalkali metal ions from the ion exchange compartment into the cathodecompartment, removing the aqueous solution of chloric acid and alkalimetal chlorate from the ion exchange compartment, and, increasing thechlorate ion concentration to provide the aqueous solution of chloricacid and alkali metal chlorate with a total chlorate ion to water ratioof about 0.15 or greater.

BRIEF DESCRIPTION OF DRAWINGS

More in detail, the novel process of the present invention and itsapplication in producing chlorine dioxide can be carried out inapparatus illustrated in the following FIGURES.

FIG. 1 is a sectional side elevational view of an electrolytic cellwhich can be employed in the novel process of the invention.

FIG. 2 is a sectional side elevational view of an additionalelectrolytic cell which can be employed in the novel process of theinvention.

FIG. 3 is a diagrammatic illustration of a system which can be employedin the process of the invention.

DETAILED DESCRIPTION OF DRAWINGS

FIG. 1 shows an electrolytic cell 4 divided into anode compartment 10,ion exchange compartment 20, and cathode compartment 30 by cationpermeable ion exchange membranes 16 and 24. Anode compartment 10includes anode 12, and anode spacer 14. Anode spacer 14 positions porousanode 12 with respect to cation permeable ion exchange membrane 16 andaids in the disengagement of anolyte gas produced. Anolyte disengager 18completes the disengagement of anolyte gas from the spent anolytesolution. Ion exchange compartment 20 includes spacer material 22 whichprovides a flow channel between cation permeable ion exchange membranes16 and 24 for the aqueous alkali metal chlorate solution. Cathodecompartment 30 includes cathode 32, and cathode spacer 34. Cathodespacer 34 positions cathode 32 with respect to cation permeable ionexchange membrane 24 and aids in the disengagement of catholyte gasproduced. The disengagement of catholyte gas from the spent catholytesolution is accomplished in cathode disengager 36.

In FIG. 2, electrolytic cell 4 has been expanded to include a second ionexchange compartment 40 which is positioned between anode compartment 10and ion exchange compartment 20. Cation permeable ion exchange membrane42 separates ion exchange compartment 20 from ion exchange compartment40. The sodium chlorate feed solution enters the lower part of ionexchange compartment 20, flows upward and out of ion exchangecompartment 20 into the upper part of ion exchange compartment 40. TheHClO₃ /NaClO₃ product solution is recovered from the lower part of ionexchange compartment 40.

The flow direction in the ion exchange compartments can also bereversed, for example, with the solution from the top of ion exchangecompartment 20 being fed to the bottom of ion exchange compartment 40.The product solution then exits from the top of ion exchange compartment40.

In the process depicted in FIG. 3, the HClO₃ /NaClO₃ solution from ionexchange compartment 20 of electrolytic cell 4 is fed to concentrator50. Water is removed from the solution by evaporation under a vacuum anda slurry of NaClO₃ crystals in a HClO₃ /NaClO₃ solution is produced. Theslurry is fed to separator 52 where the NaClO₃ crystals are removed froma concentrated solution of chloric acid and sodium chlorate as theproduct. Sodium chlorate crystals from separator 52 are recycled throughline 53 to dissolving tank 54.

DETAILED DESCRIPTION OF INVENTION

An aqueous solution of an alkali metal chlorate is fed to the single ormultiple ion exchange compartments of the electrolytic cell. Suitablealkali metal chlorates include sodium chlorate, potassium chlorate andlithium chlorate. In order to simplify the disclosure, the process ofthe invention will be described using sodium chlorate, which is apreferred embodiment of the alkali metal chlorates. As shown in FIG. 3,the sodium chlorate feed solution may be prepared, for example, bydissolving crystalline sodium chlorate in water. Commercial sodiumchlorate is suitable as it has a low sodium chloride content and theformation of undesireable amounts of chlorine dioxide in theelectrolytic cell is prevented.

Aqueous sodium chlorate feed solutions which may be employed contain anysuitable concentrations of sodium chlorate up to about saturation at thetemperatures at which the solutions are fed to the ion exchangecompartment. For example, solutions having a concentration in the rangeof from about 1% by weight to about 60% by weight of NaClO₃, preferablyfrom about 20 to about 55%, and more preferably at from about 30 toabout 50% by weight of NaClO₃, at temperatures in the range of fromabout 0° to about 100°, and preferably from about 15° to about 80° C.

The novel process of the invention utilizes an electrochemical cell togenerate hydrogen ions that displace or replace a portion of the sodiumions present in the aqueous sodium chlorate solution feed stream.

The generation of hydrogen ions in the process of the present inventionin the anode compartment is accompanied, for example, by the oxidationof water on the anode into oxygen gas and H+ ions by the electrodereaction as follows:

    2H.sub.2 O→O.sub.2 +4H.sup.+ +4e.sup.-              (4)

The anode compartment contains an anolyte, which can be an aqueoussolution of any non-oxidizable acid electrolyte which is suitable forconducting hydrogen ions into the ion exchange compartment.Non-oxidizable acids which may be used include sulfuric acid, phosphoricacid, perchloric acid and the like. Where a non-oxidizable acid solutionis used as the anolyte, the concentration of the anolyte is preferablyselected to match the osmotic concentration characteristics of thealkali metal chlorate solution fed to the ion exchange compartment tominimize water exchange between the anode compartment and the ionexchange compartment. Additionally, a solution of hydrochloric acid or amixture of hydrochloric acid and an alkali metal chloride can be used asthe anolyte, which results in a generation of chlorine gas at the anode.Where a chlorine generating anolyte is employed, it is necessary toselect a cation exchange membrane as the separator between the anodecompartment from the ion exchange compartment which is stable tochlorine gas.

The anode compartment may also employ as the anolyte electrolyte astrong acid cation exchange resin in the hydrogen form and an aqueoussolution such as deionized water.

Any suitable anode may be employed in the anode compartment, includingthose which are available commercially as dimensionally stable anodes.Preferably, an anode is selected which will generate oxygen gas.

These anodes include porous or high surface area anodes. As materials ofconstruction for the anodes metals including platinum, gold, palladium,or mixtures or alloys thereof, or thin coatings of such materials onvarious substrates such as valve metals, i.e. titanium, can be used.Additionally oxides of iridium, rhodium or ruthenium, and alloys withother platinum group or precious metals could also be employed.Commercially available oxygen evolution anodes of this type includethose manufactured by Englehard (PMCA 1500) or Eltech (TIR-2000). Othersuitable anode materials include graphite, graphite felt, a multiplelayered graphite cloth, a graphite cloth weave, carbon, and the like.

The hydrogen ions generated in the anode compartment pass through thecation exchange membrane into the sodium chlorate solution in the ionexchange compartment. As a hydrogen ion enters the solution, a sodiumion is displaced and by electrical ion mass action passes through thecation membrane adjacent to the cathode compartment to maintainelectrical neutrality.

The novel process of the invention as operated results in the conversionof sodium chlorate to chloric acid over a wide range, for example, fromabout 1 to about 99.9%, preferably from about 5 to about 95, and morepreferably from about 15 to about 90%.

The sodium chlorate feed solution concentration, the residence time inthe ion exchange compartment as well as the cell amperage are factorsthat affect the extent of the conversion of sodium chlorate to chloricacid.

Using very dilute solutions of sodium chlorate, high percentages ofconversion of NaClO₃ to chloric acid can be achieved, i.e. up to 99.9%conversion. For a single pass flow through system, typical residencetimes in the ion exchange compartment are between about 0.1 to about 120minutes, with a more preferred range of about 0.5 to about 60 minutes.

Thus the concentration of sodium chlorate in the solution fed to the ionexchange compartment and the flow rate of the solution through the ionexchange compartment are not critical and broad ranges can be selectedfor each of these parameters.

The novel process of the present invention is operated at a currentdensity of from about 0.01 KA/m² to about 10 KA/m², with a morepreferred range of about 0.05 KA/m² to about 3 KA/m².

Current efficiencies during operation of the process of the inventioncan be increased by employing additional ion exchange compartments, asillustrated by FIG. 2, which are adjacent and operated in a series flowpattern.

Adjusting the width of the ion exchange compartment can also alter theoperating cell voltage and current efficiency. The width, or spacebetween the cation exchange membranes forming the walls of the ionexchange compartment, is in the range of from about 0.1 to about 10, andpreferably from about 0.3 to about 5 centimeters.

In an alternate embodiment the ion exchange compartment contains acation exchange medium. Cation exchange mediums which can be used in theion exchange compartment include cation exchange resins. Suitable cationexchange resins include those having substrates and backbones ofpolystyrene based with divinyl benzene, cellulose based, fluorocarbonbased, synthetic polymeric types and the like. Where more than one ionexchange compartment is employed, inclusion of the cation exchangemedium is optional for each compartment.

Functional cationic groups on these mediums which may be employedinclude carboxylic acid, sulfonic or sulfuric acids, and acids ofphosphorus such as phosphonous, phosphonic or phosphoric. The cationexchange resins are suitably ionically conductive so that a practicalamount of current can be passed between the cation exchange membranesused as separators. Various percentage mixture of resins in the hydrogenform and the sodium form may be used in various sections of the ionexchange compartments on assembly to compensate for the swelling andcontraction of resins during cell operation. For example, percentageratios of hydrogen form to sodium form may include those from 50 to100%.

The use of cation exchange resins in the ion exchange compartment canserve as an active mediator which can exchange or absorb sodium ions andrelease hydrogen ions. The hydrogen ions generated at the anode thusregenerate the resin to the hydrogen form, releasing sodium ions to passinto the cathode compartment. Their employment is particularlybeneficial when feeding dilute sodium chlorate solutions as they helpreduce the cell voltage and increase conversion efficiency.

Preferred as cation exchange mediums are strong acid type cationexchange resins in the hydrogen form as exemplified by low cross-linkedresins such as AMBERLITE® IRC-118 (Rohm and Haas Co.) as well as highercross-linked resins i.e. AMBERLITE® IRC-120. High surface areamacro-reticular or microporous type ion exchange resins havingsufficient ionic conductivity in the ion exchange compartments are alsosuitable.

Physical forms of the cation exchange resin which can be used are thosewhich can be packed into compartments and include beads, rods, fibers ora cast form with internal flow channels. Bead forms of the resin arepreferred.

Cation exchange membranes selected as separators between compartmentsare those which are inert membranes, and are substantially impervious tothe hydrodynamic flow of the alkali metal chlorate solution or theelectrolytes and the passage of any gas products produced in the anodeor cathode compartments. Cation exchange membranes are well-known tocontain fixed anionic groups that permit intrusion and exchange ofcations, and exclude anions from an external source. Generally theresinous membrane or diaphragm has as a matrix, a cross-linked polymer,to which are attached charged radicals such as --SO⁻ ₃ and/or mixturesthereof with --COOH⁻. The resins which can be used to produce themembranes include, for example, fluorocarbons, vinyl compounds,polyolefins, hydrocarbons, and copolymers thereof. Preferred are cationexchange membranes such as those comprised of fluorocarbon polymers orvinyl compounds such as divinyl benzene having a plurality of pendantsulfonic acid groups or carboxylic acid groups or mixtures of sulfonicacid groups and carboxylic acid groups. The terms "sulfonic acid group"and "carboxylic acid groups" are meant to include salts of sulfonic acidor salts of carboxylic acid groups by processes such as hydrolysis.

Suitable cation exchange membranes are readily available, being soldcommercially, for example, by Ionics, Inc., Sybron, by E. I. DuPont deNemours & Co., Inc., under the trademark "NAFION®", by the AsahiChemical Company under the trademark "ACIPLEX®", and by Tokuyama SodaCo., under the trademark "NEOSEPTA®". Among these are the perfluorinatedsulfonic acid type membranes which are resistant to oxidation and hightemperatures such as DuPont NAFION® types 117, 417, 423, etc., membranesfrom the assignee of U.S. Pat. No. 4,470,888, and otherpolytetrafluorethylene based membranes with sulfonic acid groupings suchas those sold under the RAIPORE tradename by RAI Research Corporation.

The catholyte can be any suitable aqueous solution, including neutral oralkaline salt solutions, and any appropriate acids such as hydrochloric,sulfuric, phosphoric, nitric, acetic or others.

In a preferred embodiment, deionized or softened water or sodiumhydroxide solution is used as the catholyte in the cathode compartmentto produce an alkali metal hydroxide. The water selection is dependenton the desired purity of the alkali metal hydroxide by-product. Thecathode compartment may also contain a strong acid cation exchange resinin a cation form such as sodium as the electrolyte.

Any suitable cathode which generates hydrogen gas may be used, includingthose, for example, based on nickel or its alloys, includingnickel-chrome based alloys; steel, including stainless steel types 304,316, 310, and the like; graphite, graphite felt, a multiple layeredgraphite cloth, a graphite cloth weave, carbon; and titanium or othervalve metals as well as valve metals having coatings which can reducethe hydrogen overvoltage of the cathode. The cathode is preferablyperforated to allow for suitable release of the hydrogen gas bubblesproduced at the cathode particularly where the cathode is placed againstthe membrane.

Optionally a porous spacer material such as a chemically resistantnon-conductive plastic mesh or a conductive material like graphite feltcan be positioned behind the anode and/or the cathode to support theelectrodes and to permit the adjustment of the gap between the electrodeand the cation permeable ion exchange membrane, for example, when usinghigh open area expanded metal electrodes. The porous spacer materialpreferably has large holes for ease of disengagement of the gases fromthe anolyte and/or catholyte. A thin protective spacer can also beplaced between the anode and/or the cathode and the cation permeable ionexchange membranes. This spacer can be a non-conductive plastic or aporous conductive material like graphite felt. The cell may be operatedwith the electrode in contact with the thin protective spacer and theporous spacer material, or with the membrane in direct contact with theelectrode and with or without the porous spacer material.

It will be recognized that other configurations of the electrolytic cellcan be employed in the novel process of the present invention, includingbipolar cells utilizing a solid plate type anode/cathode or bipolarmembranes. For example, a bipolar electrode could include a valve metalsuch as titanium or niobium sheet clad to stainless steel. The valvemetal side could be coated with an oxygen evolution catalyst and wouldserve as the anode.

An alternative anode/cathode combination which is commercially availableis a platinum clad layer on stainless steel or niobium or titanium andis prepared by heat/pressure bonding.

The novel product solution contains chloric acid and alkali metalchlorate in a wide range of concentrations and ratios of chloric acid toalkali metal chlorate. For example, the solutions produced can providemolar ratios of chloric acid to alkali metal chlorate of from about0.1:1 to about 250:1. Where the product solutions are to be used in aknown process for the generation of chlorine dioxide, suitable molarratios of chloric acid to alkali metal chlorate of from about 0.3:1 toabout 200:1, and preferably from about 1:1 to about 100:1. Preferably,these chloric acid/alkali metal chlorate solutions are substantiallyfree of anionic or cationic impurities and contain about 30% or greaterby weight of HClO₃.

These solutions are highly acidic and can be used without requiringadditional acids, or where additional acids are used, permit asubstantial reduction in the amount of acid employed in the generationof chlorine dioxide.

Where the solutions are to be fed to a generator for chlorine dioxidewithout additional acid, it is desired that ratio of total chlorate,provided by HClO₃ and an alkali metal chlorate such as NaClO₃, to thewater present in the solution be greater than about 0.15. While therequired concentrations of HClO₃ and, for example, NaClO₃ vary somewhatwith the temperature of the solution, employing weight percent ratios ofHClO₃ to NaClO₃ of about 2.7 or greater will readily provide the desiredtotal chlorate to water ratio. Solutions of these concentrations ofHClO₃ could be produced directly in the electrochemical cell. However,it may be desirable to concentrate the HClO₃ in the product solutionsafter removal from the cell as crystals of the alkali metal chlorate maybe formed during the concentration.

The product solutions may be concentrated, for example, by evaporationat sub-atmospheric pressures and temperatures of less than about 100° C.

For example, in the range of from about 30 to about 90° C., andpreferably, from about 50° to about 80° C. solutions containing up toabout 50% by weight of chloric acid, and preferably in the range of fromabout 30 to about 40% by weight of chloric acid, may be produced in thismanner.

Freeze concentration may be used to separate sodium chlorate from thechloric acid solution and thus concentrate the remaining chloric acidsimultaneously. As the solution of chlorate and chloric acid is chilled,ice and sodium chlorate will crystallize simultaneously and in separatephases. The solution will be enriched in chloric acid and reduced inwater content until a eutectic point is reached at which chloric acid(hydrate) also crystallizes. This eutectic point is believed to occur ata solution concentration of about 30 to 35% chloric acid by weight.During the practice of freeze concentration it is also possible torecycle the melted ice along with the sodium chlorate, which dissolvesas the ice melts for the purpose of preparing new sodium chloratesolution for feed to the process of this invention. An alternateembodiment includes a combination of freeze concentration followed byvacuum evaporation to further concentrate the chloric acid.

The novel process of the present invention permits the production ofsolutions having a wide range of concentrations of chloric acid andsodium chlorate for use in chlorine dioxide generators. The productsolution can be fed directly from the electrolytic cell to a commercialchlorine dioxide generator. Typical commercial processes are those whichuse sulfuric acid or hydrochloric acid with a reducing agent such assulfur dioxide or methanol in the presence of a salt such as sodiumchloride. Commercial chlorine dioxide processes which may use theaqueous solutions of chloric acid and alkali metal chlorate of theinvention include the Mathieson, Solvay, R2, R3, R8, Kesting, SVP, andSVP/methanol, among others.

To increase yields of chlorine dioxide and conversion efficiencies it ispreferred to carry out the process in the presence of a solid surfacewhich promotes oxygen evolution. Any solid surface may be used whichfacilitates oxygen formation including oxygen-evolving catalysts.Suitable as oxygen-evolving surfaces or catalysts are, for example,metals and oxides of the elements of Group VIII of the Periodic Table ofElements (Webster's Third New International Dictionary of the EnglishLanguage. Unabridged. 1986, p. 1680). Thus metals such as the platinumgroup metals including platinum, palladium, iridium, rhodium orruthenium; and mixtures or alloys of these platinum group metals may beemployed. Additionally oxides of platinum group metals such as iridium,rhodium or ruthenium, as well as mixtures of these oxides with platinumgroup metals or alloys of these precious metals could be suitablyemployed. Likewise, iron alloys such as stainless steel, nickel ornickel based alloys, and cobalt based alloys can be used asoxygen-evolving catalysts in the process of the invention. Otheroxygen-evolving catalysts include semiconductive ceramics known asperovskites. The catalyst may be present as particles suspended in thereaction mixture or supported on an inert substrate. The oxygen-evolvingcatalysts may be used in the forms of a packed bed, slurries, or anystructure which will suitably promote mass transfer. In a preferredembodiment of this invention, the catalyst is supported on valve metalheat exchanger surfaces to facilitate evaporation of water during thereaction. Suitable valve metals include titanium and tantalum, amongothers.

The process permits flexibility in any by-product salts which may beproduced as well as allowing the recovery of energy costs by producing,for example, an alkali metal hydroxide solution by-product. Further theprocess reduces operating costs by eliminating process steps andequipment from processes presently available.

To further illustrate the invention the following examples are providedwithout any intention of being limited thereby. All parts andpercentages are by weight unless otherwise specified.

EXAMPLE 1

An electrochemical cell of the type shown in FIG. 1 consisting of threecompartments machined from ultra high density polyethylene (UHDPE)including an anode compartment, a central ion exchange compartment, anda cathode compartment. The 1/2 inch (1.27 cm) thick anode compartmentcontained a titanium mesh anode having an oxygen-evolving anode coating(PMCA 1500® Englehard Corporation, Edison, N.J.). The anode wassupported and spaced apart from the UHDPE back wall using multiplelayers of polyethylene mesh having 1/4 inch square holes and being 1/16inch in thickness. A polyethylene mesh spacer was positioned between theanode and adjoining membrane to provide an anode-membrane gap of 0.0625inch (0.1588 cm). The anode compartment was filled with a 2.0 percent byweight sulfuric acid solution. The 1/2 inch (1.27 cm) thick cathodecompartment contained a 304 stainless steel perforated plate cathodemounted flush to the surface of the cathode compartment with thepolyethylene mesh spacers. The cathode was positioned in contact withthe adjacent membrane providing a zero distance gap. The cathodecompartment was initially filled with a sodium hydroxide solution (2% byweight) as the catholyte. Separating the anode compartment from the ionexchange compartment, and the ion exchange compartment from the cathodecompartment were a pair of perfluorosulfonic acid cation permeablemembranes with a 985 equivalent weight, obtained from the assignee ofU.S. Pat. No. 4,470,888. The ion exchange compartment was a machined 1/4inch (0.625 cm) thick frame with inlet and outlet and contained thepolyethylene mesh spacers to distribute the chlorate solution as well asto support and separate the two membranes. An aqueous sodium chloratesolution containing 20 weight percent of NaClO₃ was prepared bydissolving reagent grade sodium chlorate in deionized water.

During operation of the electrolytic cell, the chlorate solution wasmetered into the bottom of the ion exchange compartment in a single passprocess at feed rates varying from 7.0 g/min. to 14.4 g/min. Electrolytecirculation in the anode and cathode compartments was by gas lift effectonly. The cell was operated employing a cell current of 24.5 amperes ata current density of 1.20 KA/m². The cell voltage varied according tothe cell operating temperature. A sample of the product solution wastaken at each flow rate, the temperature measured, and the productsolution analyzed for chloric acid and sodium chlorate content. Theproduct solutions were colorless, indicating no chlorine dioxide wasformed in the ion exchange compartment. The concentration of the sodiumhydroxide catholyte during cell operation increased to 12 percent byweight. The results are given in Table I below.

                                      TABLE I                                     __________________________________________________________________________            NaClO.sub.3 Feed                                                                           HClO.sub.3 --NaClO.sub.3 Product                         Cell                                                                              Cell                                                                              Flowrate                                                                             Product                                                                             HClO3                                                                             NaClO3                                                                             HClO3:NaClO3                                                                          Conversion                                                                            C.E.                                                                             Residence                                                                            KWH/TON               Volts                                                                             Amps                                                                              (gm/min)                                                                             Temp (C.)                                                                           Wt %                                                                              Wt % Molar Ratio                                                                           % to HClO3                                                                            %  Time (min)                                                                           of                    __________________________________________________________________________                                                            HClO3                 5.00                                                                              24.5                                                                              14.40  30.0  5.96                                                                              12.49                                                                              0.60    38.00   69.50                                                                            11.38  2082                  4.87                                                                              24.5                                                                              12.35  42.0  6.51                                                                              11.80                                                                              0.70    41.00   65.20                                                                            13.27  2152                  4.76                                                                              24.5                                                                              10.00  45.0  7.24                                                                              10.88                                                                              0.84    45.60   58.60                                                                            16.39  2336                  4.50                                                                              24.5                                                                              7.17   50.0  8.34                                                                              9.49 1.11    52.60   48.50                                                                            22.86  2674                  4.44                                                                              24.5                                                                              7.00   54.0  8.43                                                                              9.38 1.13    53.10   47.80                                                                            23.41  2673                  __________________________________________________________________________

EXAMPLE 2

The electrochemical cell of FIG. 2 was employed having a second ionexchange compartment adjacent to the first ion exchange compartment. Theanode compartment containing the same type of anode used in Example 1was filled with a strong acid hydrogen form cation exchange resin(AMBERLITE® IRC-120 plus, Rohm & Haas Company) as the electrolyte. Aperfluorinated sulfonic acid-based membrane (Dupont NAFION® 417)separated the anode compartment from the first ion exchange compartment.The two ion exchange compartments were fully filled with AMBERLITE®IRC-120 plus cation exchange resin in the hydrogen form and wereseparated by a Dupont NAFION® 417 membrane. The same membrane wasemployed to separate the second ion exchange compartment from thecathode compartment. The cathode compartment contained a perforated 304stainless steel cathode, and was filled with a sodium form AMBERLITE®IRC-120 plus cation exchange resin. Both the anode compartment and thecathode compartment were filled with deionized water. The sodiumchlorate solution fed to the ion exchange compartments was prepared fromreagent grade sodium chlorate dissolved in deionized water to form a 16weight percent solution as sodium chlorate. The sodium chlorate solutionat 20° C. was fed to the bottom of ion exchange compartment 20 adjacentto the cathode compartment at a flow rate of 6.5 grams per minute. Thechloric acid-sodium chlorate solution flow from the upper part of ionexchange compartment 20 was routed into the bottom of ion exchangecompartment 40 adjacent to the anode compartment and collected from thetop of ion exchange compartment 40. The total residence time of thesolution in the ion exchange compartments was about 42 minutes.

During operation of the cell, the cell current was set at a constant23.0 amperes for an operating current density of 1.5 KA/m². The cellvoltage stabilized at 9.60 volts and the product temperature was 65° C.

Circulation in the anode and cathode compartments of the electrolyte wasby gas lift effect and the liquid level of the gas disengagers was setat 3 inches (7.62 cm) above the height of the cell.

The product solution from the cell contained 11.44 weight percent asHClO₃ which represented a 90% conversion of the sodium chlorate tochloric acid. The current efficiency was determined to be 61.6% and thepower consumption was 4490 KWH/Ton of HClO₃. The product solution waslight yellow in color, indicating the presence of some chlorine dioxideor chlorine in the chloric acid-sodium chlorate solution product.

EXAMPLE 3

An electrochemical cell was constructed similar to that of FIG. 1consisting of three compartments. The anolyte and catholyte compartmentswere machined from 1 inch (2.54 cm) thick natural polyvinylidenedifluoride (PVDF). The outside dimensions of both the anolyte andcatholyte compartments were 5 inches (12.7 cm) by 14 inches (35.56 cm)with machined internal dimensions of 3 inches (7.62 cm) by 12 inches(30.48 cm) by 0.250 inch (0.635 cm) deep. Flow entry and exit ports aswell as flow distributions holes were drilled from the outside of theframe to the central recess area for flow into and out of thecompartments.

The central ion exchanging compartment was machined from 1 inch (2.54cm) thick natural PVDF with outside dimensions of 5 inch (12.7 cm) by 16inch (40.64 cm) to a 1/8" (0.317 cm) by 5 inch (12.7 cm) by 14 inch(35.56 cm) thick center area with a central 3 inch (7.62 cm) by 13 inch(33.02 cm) area cutout. The 1 inch (2.54 cm) by 1 inch top and bottomends were drilled with a single central hole to form entry/exit portsand tapped to accept 1/4 inch NPT pipe thread fittings. A series of0.055 inch (0.1397 cm) holes were drilled every 1/2 inch (1.27 cm) fromthe central cutout area into the flow distribution hole in the 1 inch by1 inch entry/exit ends of the ion exchanging compartment frame.

These flow distribution holes go through the 1/8 inch thickness of thecentral part of the frame.

The anolyte compartment was fitted with an open diamond patternnonflattened expanded sheet prepared from 0.060 inch (0.1524 cm) thicktitanium with the expanded metal sheet having a total depth of 0.140inches (0.3556 cm) by 3 inch (7.62 cm) wide by 12 inch (30.48 cm) longdimensions. A 1/2 inch (1.27 cm) wide by 12 inch (30.48 cm) long by 1/16inch (0.0625 cm) thick titanium current distributor strip was welded atmultiple points to the backside of the expanded titanium sheet.

Two 1/2 inch (1.27 cm) diameter titanium current conductor posts werethen welded to the back side of the flat titanium current distributionstrip. The expanded titanium surfaces were then plated with a layer ofmetallic platinum approximately 2 micron (78 microinch) thick by a brushelectroplating method using a diluted chloroplatinic acid solution. Theanode structure was then mounted into the recess inside the anolytecompartment using one or more layers of 1/16 inch (0.1588 cm) thickexpanded polytetrafluorethylene mesh behind the anode to make the anodesurface flush with the inside surface of the anolyte compartment.

The catholyte compartment was fitted with a 1/16 inch (0.1588 cm) thickby 3 inch (7.62 cm) by 12 inch (30.48 cm) type 316L stainless steelperforated plate having two 1/2 inch (1.27 cm) diameter 316L stainlessconductor posts welded on the back side. The cathode plate was mountedin the recess inside the catholyte compartment using two layers of 1/16inch (0.1588 cm) thick expanded polytetrafluorethylene mesh behind thecathode plate to make the cathode surface flush with the inside surfaceof the catholyte compartment.

The electrochemical cell assembly was completed using 0.040 inch (0.1016cm) thickness polytetrafluorethylene compressible GORE-TEX gasket tape(W. L. Gore & Associates, Elkton, Md.) on the sealing surfaces of allthe compartment cell frames.

Two layers of a loose woven polytetrafluoroethylene filament with a 1/16inch (0.1588 cm) thickness were laid in place in the central ionexchange compartment to provide for flow distribution and physicalformation of the flow channel. DuPont NAFION 324 perfluorinated sulfonicacid cation permeable type membranes were then mounted between thecentral ion exchange compartment and the anolyte and catholytecompartments.

The above cell was operated with a reagent grade 47.15 wt % sodiumchlorate solution fed at a flowrate of 11.17 gm/min into the bottom ofthe central ion exchange compartment. Deionized water was metered intothe bottom of the catholyte compartment at a flowrate of 10.40 gm/min.The anolyte was a 30 wt % sulfuric acid which was recirculated at aflowrate of about 50 gm/min with a pump. The applied cell current was 70amperes for a current density of 3 KA/m2 and the cell voltage was 5.55volts. A slightly yellow product solution exited the central ionexchanging compartment at a temperature of about 80° C. and, uponanalysis, had a composition of about 18.17 wt % HClO₃, 26.47 wt %NaClO₃, (total as ClO⁻ ₃, 38.71 wt %) and HClO₄ content of less than0.03 wt %. The catholyte product contained 7.33 wt % NaOH at an outputflowrate of about 12.14 gm/min. The calculated cell operating currentefficiency based on the NaOH produced was 51.1%, and 51.7% based on theHClO₃ present in the product.

Chloric acid/sodium chlorate solution products from the above cellduring several runs provided a composite solution containing about 20.0wt % HClO₃ and about 22 wt % NaClO₃, [equivalent to a total chlorate ion(ClO⁻ ₃) content of about 37.0 wt %]. The total chlorate (ClO⁻ ₃)content was determined by a standard iodiometric method usingconcentrated HCl and titration with thiosulfate.

The perchloric acid (HClO₄) concentration was determined by ionchromatography using dissolved sodium perchlorate solutions asstandards. The sodium chlorate content of the samples were calculated bydifference.

About 500 ml of the above clear, water color solution was placed in a 1liter vacuum filter flask fitted with a thermometer, polyfluoroethylenecoated magnetic stirring bar, and a 28 mm Hg vacuum water aspiratorsource. The flask was then placed on a hot plate/magnetic stirrer andheated while the vacuum was applied. The solution was vacuum evaporatedin a temperature range between 50°-80° C. Approximately 50 to 100 mlsamples of the solution were periodically removed during the vacuumevaporation process. The first three samples showed no precipitates inthe hot solution on sampling. The fourth and final sample was obtainedwhen a significant amount of sodium chlorate crystals had accumulated inthe vacuum flask at 60° C. and there was a slight yellow color in thesolution phase.

All of the samples were then cooled in stoppered flasks in a cold waterbath to 20° C. for a period of 1.5 hours to allow the solution to cometo equilibrium with the precipitated NaClO₃ salt phase. Samples of thewater colored supernatants were then removed for analysis. The chloricacid (HClO₃) content was determined by titration with NaOH.

The results are listed in Table II below.

                  TABLE II                                                        ______________________________________                                                   HClO.sub.3                                                                            ClO.sub.3 .sup.-                                                                         NaClO.sub.3                                                                          HClO.sub.4                               Sample No. wt %    wt %       wt %   wt %                                     ______________________________________                                        1          21.33   37.82      21.36  <0.03                                    2          25.26   38.42      17.17  <0.03                                    3          29.75   40.04      13.57  <0.03                                    4          35.85   42.73      9.32   <0.03                                    ______________________________________                                         Note:                                                                         Ion Chromatography Detection Limit for HClO.sub.4 was 0.03 wt %.         

EXAMPLE 4

The apparatus of Example 3 was used to produce about 150 gm of a HClO₃/NaClO₃ solution having a composition of about 19.24 wt % HClO₃ and27.64% NaClO₃ with a total ClO⁻ ₃ content of 40.68 wt %. The solutionwas evaporated at temperatures between 40°-60° C. under a vacuum of28-30 mm Hg. Sodium chlorate crystals precipitated in the solution insignificant amounts after 1/3 of the solution had been evaporated. Thesolution phase had a slight yellow color toward the end of the hotevaporation and disappeared upon cooling the solution to roomtemperature with applied vacuum. The solution phase from the flask wasdecanted from the solids and cooled in a stoppered flask in an ice bathto 5° C. The solution phase was then decanted from the sodium chloratecrystal solids and analyzed.

The solution phase contained 43.02 wt % as HClO₃ and 45.24 wt % as ClO⁻₃. This calculates to about 3.48 wt % as NaClO₃. Ion chromatographyanalysis showed that the HClO₄ content of the solution was <0.03 wt %. A2.025 gm sample of the product solution was carefully evaporated todryness on a hot plate to about 250° C. for an actual sodium chlorateresidual analysis check. The final weight of the sodium chlorate depositresidual in the sample was found to be 0.065 gm or about 3.21 wt % asNaClO₃. This is in close agreement with the calculated 3.48 wt % value.

COMPARATIVE EXAMPLE A

About 150.00 gm of a solution containing 19.24 wt % HClO₃ and 27.64 wt %NaClO₃, as was used in Example 4, was evaporated by boiling in a 250 mlbeaker on a hot plate. The solution boiled at an increasing value froman initial 105°-107° C. to about 115° C. at the end of the run. Afterboiling the solution had a deep yellow color and the final weight of thesolution was 116.35 gm.

After cooling to room temperature sodium chlorate solids precipitated.About 46.88 gm of liquid was decanted from the sodium chlorate solids.The liquid decant was then further cooled in an ice bath to 5° C. Theclear decant was analyzed and found to contain about 27.88 wt % HClO₃and 1.66 wt % HClO₄ by ion chromatography. The residual NaClO₃ contentof about 13.47 wt % was determined by evaporating a 2.005 gm sample ofthe liquid to obtain 0.270 gm of solids.

EXAMPLE 5

About 300 ml of HClO₃ /NaClO₃ product solution having a composition ofabout 19.24 wt % HClO₃ and 27.64% NaClO₃ with a total 40.68 wt % ClO⁻ ₃were evaporated at 28-30 mm Hg vacuum at temperatures between 40°-60° C.Liquid samples of the evaporated solution were taken from the flaskduring the evaporation process. In examples 5-8 the solution was cooledto 15° C.; in example 9 the solution was cooled to 5° C.; and in example10 the solution was cooled to 0° C. After cooling, the solution phasewas decanted from the precipitated sodium chlorate crystals andanalyzed.

The results, given in Table IIIA below, show that the sodium ion i.e.Na+ content, in the chloric acid can be significantly reduced to levelsbelow 1% as Na+. The highest chloric acid concentration achieved in was51.39 wt %.

Table IIIB below shows the published solubility limits of sodiumchlorate at 15°, 20° and 25° C. As can be seen, at 15° C., a saturatedsodium chlorate solution has a ClO⁻ ₃ /H₂ O molar ratio of 0.155. Asshown by examples 2, 3, 4, 5 and 6 in Table IIIA, HClO₃ /NaClO₃ productsolutions of the invention are produced having a higher ClO⁻ ₃ /H₂ Omolar ratio than that of the saturated sodium chlorate solution.

Thus, the advantage with the HClO₃ /NaClO₃ solutions of the inventionhaving ClO.sup.₃ /H₂ O ratios greater than about 0.155 is that there ismore chlorate ion (ClO⁻ ₃) available for chlorine dioxide production perweight or volume of solution than in the typically used commercialsodium chlorate solutions, as well as less sodium ion and less water.

                                      TABLE IIIA                                  __________________________________________________________________________           Solution                                                                           HClO3                                                                             ClO3.sup.-                                                                        NaClO3                                                                             HClO4                                                                             Na.sup.+                                                                          HClO.sup.3                                                                         ClO3.sup.-                                                                         ClO3.sup.- /H2O                                                                      ClO3.sup.- /H2O             Example No.                                                                          Sp. Gr.                                                                            Wt %                                                                              Wt %                                                                              Wt % Wt %                                                                              Wt %                                                                              Molarity                                                                           Molarity                                                                           Wt Ratio                                                                             Molar                       __________________________________________________________________________                                                      Ratio                        5*    1.3230                                                                             20.50                                                                             37.74                                                                             22.30                                                                              <0.03                                                                             4.82                                                                              3.21 5.98 0.660  0.142                        6*    1.3161                                                                             32.65                                                                             41.14                                                                             11.33                                                                              <0.03                                                                             2.45                                                                              5.09 6.49 0.734  0.159                        7*    1.3309                                                                             35.29                                                                             42.70                                                                             9.99 <0.03                                                                             2.16                                                                              5.56 6.81 0.780  0.168                        8*    1.3474                                                                             42.04                                                                             46.52                                                                             6.35 <0.03                                                                             1.37                                                                              6.71 7.51 0.901  0.195                        9**   --   42.72                                                                             45.30                                                                             2.58 --  0.56                                                                              --   --   0.828  0.179                       10**   --   51.39                                                                             52.50                                                                             1.42 --  0.31                                                                              --   --   1.113  0.240                       __________________________________________________________________________     *Decanted product solution cooled to 15° C.                            **Decanted product solution cooled to 5° C.                            ***Decanted product solution cooled to 0° C.                      

                                      TABLE IIIB                                  __________________________________________________________________________    PUBLISHED SOLUBILITY OF SODIUM CHLORATE:                                               Solution                                                                           NaClO3                                                                             ClO3.sup.-                                                                        ClO2.sup.-                                                                         ClO3.sup.- /H2O                                                                      ClO3.sup.- /H2O                            Temperature °C.                                                                 Sp. Gr.                                                                            Wt % Wt %                                                                              Molarity                                                                           Wt Ratio                                                                             Molar Ratio                                __________________________________________________________________________    15       1.4060                                                                             47.80                                                                              37.50                                                                             6.314                                                                              0.718  0.155                                      20       --   48.90                                                                              38.30                                                                             --   0.750  0.162                                      25       1.4240                                                                             50.00                                                                              39.20                                                                             6.689                                                                              0.784  0.169                                      __________________________________________________________________________

What is claimed is:
 1. An aqueous solution of chloric acid and an alkalimetal chlorate in which the concentration of chloric acid is about 30%or greater by weight of HClO₃ and the molar ratio of chloric acid toalkali metal chlorate is no more than 250:1.
 2. The aqueous solution ofclaim 1 wherein the molar ratio of chloric acid to alkali metal chlorateis no more than about 200:1.
 3. The aqueous solution of claim 2 whereinthe molar ratio of chloric acid to alkali metal chlorate is from about1:1 to about 100:1.
 4. The aqueous solution of claim 1 wherein theconcentration of chloric acid is from about 30% to about 50% by weightof the solution.
 5. The aqueous solution of claim 1 wherein said alkalimetal chlorate is sodium chlorate.
 6. The aqueous solution of claim 5wherein the weight percent ratio of HClO₃ to NaClO₃ is greater thanabout 2.7.
 7. An aqueous solution of chloric acid and an alkali metalchlorate being substantially free of anionic and cationic impurities inwhich the concentration of chloric acid is about 30% or greater byweight of HClO₃ and the molar ratio of chloric acid to alkali metalchlorate is no more than 250:1.
 8. The aqueous solution of claim 7wherein the molar ratio of chloric acid to alkali metal chlorate is nomore than about 200:1.
 9. The aqueous solution of claim 8 wherein themolar ratio of chloric acid to alkali metal chlorate is from about 1:1to about 100:1.
 10. The aqueous solution of claim 7 wherein theconcentration of chloric acid is from about 30% to about 50% by weightof the solution.
 11. The aqueous solution of claim 10 wherein saidalkali metal chlorate is sodium chlorate.
 12. The aqueous solution ofclaim 11 wherein the weight percent ratio of HClO₃ to NaClO₃ is greaterthan about 2.7.
 13. An aqueous solution of chloric acid and sodiumchlorate being substantially free of anionic and cationic impurities inwhich the concentration of chloric acid is from about 30% to about 50%and the molar ratio of chloric acid to sodium chlorate is from about 1:1to about 100:1.
 14. The aqueous solution of claim 13 wherein the weightpercent ratio of HClO₃ to NaClO₃ is greater than about 2.7.